Synthetic oils produced through a Fischer-Tropsch reaction are high in cetane number and contain almost no sulfur or aromatic and thus have been highly expected to be used as clean fuel for compression ignition engines. However, the synthetic oils produced through a Fischer-Tropsch reaction are rich in paraffins and smaller in lower heating value per unit volume than the conventional petroleum-based diesel fuels, possibly leading to a reduction in the fuel consumption. There has been no known method for suppressing reduction of the fuel consumption associated with using the Fischer-Tropsch synthetic oil from without adversely affecting the exhaust gas.